The United States says it has identified more than 40 sites in Afghanistan where Osama Bin Laden's al-Qaeda network might have been carrying out research into weapons of mass destruction.

Head of the US Central Command, General Tommy Franks, said many of the sites were in areas now under the control of anti-Taleban forces and US teams were examining them.

We will not leave weapons of mass destruction in Afghanistan

General Tommy Franks

General Franks said if any weapons of mass destruction were found, they would be eradicated.

Some of the sites included laboratories where "paraphernalia, a variety of chemical compositions and these sorts of things" were found, said General Franks, adding it was too early to tell whether or not they were being used for legitimate purposes.

Speaking during a visit by US Defence Secretary Donald Rumsfeld to Central Command headquarters in Tampa, Florida, General Franks told reporters the sites will be visited "systematically... until we have gone through all of them and performed the analyses that we need to perform to ensure ourselves that we do not have evidence of WMD [weapons of mass destruction]".

He said suspect materials would be taken to the United States for examination.

Nuclear claim

The BBC's Jonny Dymond in Washington says the US administration has made it clear that it takes seriously the claim by Osama Bin Laden that his al-Qaeda network has tried to develop weapons of mass destruction.

Bin Laden is reported to have told a newspaper he has nuclear capability

Earlier this month, Bin Laden, the main suspect in the 11 September attacks on America, told a Pakistani newspaper his group possessed chemical and nuclear weapons.

The Dawn newspaper quoted Bin Laden as saying: "If America used chemical and nuclear weapons against us, then we may retort with chemical and nuclear weapons. We have the weapons as a deterrent."

More recently, the US confirmed documents found in a building in Kabul believed to have been used as a safe house by al-Qaeda fighters contained instructions on how to build a nuclear device

As well as explaining how to trigger a thermonuclear reaction, the papers are also said to have contained formulae for producing a lethal biological poison, ricin.